【英语时差8,16】地球同步卫星：速度和轨道-下（在线收听）

 A satellite has to move at a certain speed to balance out gravity; otherwise it won’t stay in orbit. Now, a geo must move at the speed the earth turns. If you put it in a very low altitude, that speed wouldn’t be enough to balance gravity. Go higher, though, and gravity gets weaker. At 22,500 miles, gravity will be just weak enough so that a satellite moving at the speed of the earth’s rotation will stay in orbit without help. Of course, if you wanted to have a geo at a different altitude, you could always attach rockets to it and have them push continuously. But that’s a pretty expensive option. At 22,500 miles, nothing else is required.